Recently, a distance measuring apparatus has been used as a device for determining the distance to an object (i.e., the object of range finding) based on a parallax between a pair of optical systems. Specifically, such a device has been used to determine the distance between running cars and as a member of an autofocusing system for cameras or a three-dimensional shape measuring system.
Such a distance measuring apparatus includes a pair of lenses that are arranged side by side either horizontally or vertically and an imaging element with a pair of image capturing areas that are associated with the lenses, respectively. The pair of lenses produce images on their associated image capturing areas, and the distance to the object can be determined by carrying out triangulation based on the parallax between images that include images of the object which are obtained by the imaging element.
FIG. 21 illustrates the principle of triangulation carried out in a distance measuring apparatus. FIG. 21 shows a first optical system with an image capturing lens La and a second optical system with an image capturing lens Lb. These optical systems are arranged such that the optical axis Aa of the first optical system and the optical axis Ab of the second optical system run parallel to each other with a predetermined interval B left between them. A line segment that extends between the intersection of the optical axis Ab of the second optical system and an image capturing plane Nb and the intersection of the optical axis Aa of the first optical system and an image capturing plane Na is called a “base line”, which is a line segment that never varies according to the position of the object and is used as a reference for triangulation. The length of that base line is equal to the interval B. Thus, the base line length will be identified herein by “B”.
An image of the object U is formed by the image capturing lens L1 on the image capturing plane Na, while another image of the object U is formed by the image capturing lens L2 on the image capturing plane Nb. In FIG. 21, a point P on the object U is supposed to be a measuring point. If the point P is located on the optical axis Aa of the first optical system, an image of the point P will be focused at the intersection of the image capturing plane Na and the optical axis Aa of the first optical system. On the image capturing plane Nb, on the other hand, another image of the point P will be focused at a position which is distant by distance Δ from the intersection of the image capturing plane Nb and the optical axis a2 of the second optical system. This distance is called a “parallax” and its amount is called the “parallax amount Δ”.
Supposing the focal length of the image capturing lenses La and Lb of the first and second optical systems is identified by f, the following approximation equation is satisfied:
                    [                  Formula          ⁢                                          ⁢          1                ]                                                            Δ        ≈                  B          ·                      f            Z                                              (        1        )            The images focused on the image capturing planes Na and Nb are subjected to correction, division and other kinds of processing so as to be processed easily by computational processing. By making pattern matching between the images focused on the image capturing planes Na and Nb after they have been subjected to those kinds of processing, the amount of parallax Δ can be obtained. And by substituting the amount of parallax Δ calculated, the base line length B and the focal length f into Formula (1), the distance Z can be obtained.
In a distance measuring apparatus, the range finding accuracy increases as the parallax amount Δ increases. According to Formula (1), as the base line length B and the focal length f increase, the parallax amount Δ also increases, and accordingly, the range finding accuracy increases.
Patent Document 1 discloses a distance measuring apparatus that uses a positive meniscus simple lens, having a convex surface on its object plane, to increase the focal length without increasing the overall length of the lens.